When it comes to describing the Chicago Metropolitan Water Reclamation District’s (MWRDGC) Tunnel and Reservoir Protection Plan (TARP, also know as “Deep Tunnel”), superlatives abound. Once touted “the largest undertaking of its kind ever attempted by mankind,” Deep Tunnel was designed to protect Lake Michigan, the region’s drinking water supply, from raw sewage pollution; improve water quality of area rivers and streams; and provide an outlet for floodwaters to reduce street and basement sewage backup flooding. Phase I of the Deep Tunnel project is comprised of four different tunnel systems: Mainstream, Des Plaines, Calumet and Upper Des Plaines.

“The largest tunnel system is the combined Mainstream and Des Plaines system—there’s about 1.6 billion gal of storage capacity in these two tunnels and we’re currently constructing the McCook reservoir, which is going to be tied into this system,” said Henry Marks, chief operating engineer II of the Stickney Water Reclamation Plant (WRP). The largest wastewater treatment facility in the world, it serves 2.38 million people in a 260-sq-mile area that includes Chicago and 43 suburban communities. “There’s still a lot more rock to build.”

After a storm event, pumping stations dewater the tunnel systems as WRP capacity becomes available, freeing the tunnel and reservoir capacity for the next storm event. All captured combined sewer overf low (CSO) that is pumped to the WRP receives full secondary treatment prior to discharge into the waterway, in accordance with National Pollutant Discharge Elimination System regulations. Construction of the Phase I tunnel system began in 1975. The total tunnel system consists of 109.4 miles of deep rock tunnels providing 2.3 billion gal of volume to capture CSOs previously discharged at hundreds of outfall locations.

“What we’re concentrating on right now is reservoir construction,” Marks said. “A lot of this reservoir construction is also flood control. It’s being partially funded by the Army Corps of Engineers. [The U.S. Environmental Protection Agency (EPA)] gave us a lot of the funding for the original tunnels because it was [the] more pollution control portion of design of the system.”

A Look Back

After forming in 1889, the Chicago Sanitary District’s first achievement was to reverse the flow of the Chicago River away from Lake Michigan via construction of the Chicago Sanitary Ship Canal, which runs from the Chicago River to the Des Plaines River.

In an effort to intercept overflows, TARP was adopted in 1972 when contaminants thrived in local rivers, canals and Lake Michigan. The problem originated from the fact that Chicago and its older suburbs are served by combined sewers, therefore, both sanitary and storm flow are conveyed through the same pipe. As the region developed, the amount of rainwater entering the sewer systems dramatically increased. During rain events, sewer systems and treatment plants could not handle the additional flow, therefore, within the combined sewer areas, the outfalls released CSOs into waterways.

“The TARP project was unprecedented when it was adopted and is still the largest-scale CSO control project in the world,” said David St. Pierre, executive director of MWRDGC. “As portions of the tunnels were completed, they were made operational, allowing for the capture of over a trillion gallons of CSOs to date. Due in large part to TARP, the area waterways are cleaner than they have been in over a century.”

The Deep Tunnel actually is a series of tunnels up to 33 ft in diameter. Along with its large storage reservoirs, the system will store 20 billion gal of overflow until it can be treated by the district’s plants.

At the End of the Tunnel

The reservoir-focused remainder of the TARP project will progress in stages, with milestone dates targeted for 2015, 2017 and, finally, 2029.

The reservoir system consists of three reservoirs. The McCook Reservoir currently is under construction and, when complete, will boast a total capacity of 10 billion gal. It will provide more than $90 million per year in flood-damage reduc- tion benefits to 3.1 million people in 37 communities, according to MWRDGC.

The Thornton Reservoir provides overbank flood relief for nine communities and has captured more than 26 billion gal of floodwater. The second stage of its construction involves creating a permanent 7.9-billion-gal combined Natural Resources Conservation Service/Chicagoland Underflow Plan reservoir—dubbed the Thornton Composite Reservoir—to be located in the North Lobe of the Thornton Quarry. It is due for completion by 2015 and will provide $40 million per year in benefits to 556,000 people in 15 communities. Today, the Thornton Transitional Reservoir can hold 3 billion gal of storm water runoff, which is pumped to the Calumet WRP for treatment.

The Majewski Reservoir near O’Hare International Airport in Chicago already is complete and can capture up to 325 million gal of storm water overflow.

As the project continues, so do the challenges. “One of the biggest challenges is to coordinate and schedule several different construction and mining activities that need to take place within the two reservoirs by different contractors,” St. Pierre said. “The rock that is mined to create the reservoir holes is processed and sold in huge quantities as construction aggregate, and the rate at which it is mined is highly dependent on the construction market. Certain construction activities cannot proceed until the mining is completed in some areas, so we do our best to forecast when these activities can occur and make adjustments as needed depending on the mining progress.”

A sign of current financial times, funding is hard to come by.

“[Another] challenge is funding the remaining TARP projects: the McCook and Thornton Reservoirs,” St. Pierre said. “The Army Corps of Engineers was authorized to design and construct the reservoirs several years ago, with the MWRDGC acting as local sponsor. Due to so much competition for federal funds, the Corps and the district need to work each year to get enough money budgeted by Congress to keep the project on schedule. Due to limitations in the Corps’ funding, the district decided to advance the design and construction of the Thornton Reservoir on its own, so that the Corps could concentrate its funds on the McCook Reservoir.”

The MWRDGC also has had to deal with criticism over the issue of chronic CSOs, according to a March 2011 Chicago Tribune report. Still, it is important to recognize the marvelous engineering feat that is TARP, and its efforts have not gone unnoticed. It won the American Society of Civil Engineers award for most outstanding Civil Engineering Project of 1986 and was named by the EPA as one of the nation’s top Clean Water Act success stories. It undoubtedly will continue to make headlines in the coming years.